This application is a continuation-in-part of U.S. patent application Ser. No. 368,425, filed Apr. 14, 1982.
This invention relates generally to sensors for detecting very small displacements of a surface, and, more particularly, the invention relates to an improved surface acoustic wave sensor which employs an optical fiber interferometer.
The invention is primarily useful as a surface acoustic wave detector of greatly improved sensitivity over the surface acoustic wave detectors of the prior art. A surface acoustic wave is a wave representing vibration which is propagating through a solid and causes the surface of the solid to undulate.
The invention also has application in other fields as well. Because the invention described herein can detect the amplitude and frequency of the displacement of a reflecting surface, it is apparent that the invention can also be used in any other application where it is necessary to detect very small movements of a surface. Typically, the displacements of the surface which can be sensed by the invention described herein can range as low as 0.0003 angstroms. An angstrom is 10.sup.-10 meters.
As an example, the invention could be used as a very sensitive modulator to change the phase of one light wavetrain relative to another wavetrain in proporation to the small displacements of a reflecting surface such as a mirror which is mechanically moved in accordance with the modulating signal. The invention might also be used to sense the relative flatness of revolving disk support surfaces used to support magnetic storage disks used for bulk storage in computer applications. Such disks must not wobble as they spin at high RPM's lest the flying heads floating over the disks crash into the surface and ruin the head and the disk. The invention could also be used as a microphone to pick up sound waves traveling in air or water where the sound waves are extremely weak but are capable of moving a reflecting surface.
A principal application of the present invention is to acquire detailed knowledge of the propagation characteristics such as frequency and amplitude of acoustic waves traveling through solids. Such information can be used in evaluating other devices such as surface acoustic wave delay lines, filters and sonic wave guides. Further, the invention can be used to test parts for cracks since the surface acoustic wave propagation characteristics change around the vicinity of cracks in the sample surface. Very tiny cracks not visible to the naked eye can be detected in this manner in critical high stress parts such as turbine blades for jet engines and various parts in the engine and power train of automobiles.
Prior art apparatus for measuring surface acoustic wave propagation characteristics were of the bulk optic variety in that they required lasers to beam coherent beams of light toward the sample surface. The beams were reflected by the surface and converted to electrical signals by some form of detector. Such prior art systems were very inconvenient because of the necessity of bringing the laser to the workpiece which might be located in a manufacturing area having temperature and vibration conditions which are not conducive to precise control necessary for the successful operation of such systems. Such random, uncontrollable environmental phenomena such as temperature changes vibration and electromagnetic noise from motors in machines in manufacturing areas can cause problems in precisely aligning the optical equipment and electromagnetic noise problems in the sensitive electronic equipment which is used in such systems. Such equipment is used to convert the optical data to signals which can be interpreted to derive the propagation characteristics of the surface displacements of interest.